This invention relates to a focusing apparatus used in a transmission electron microscope.
As a focusing device in a transmission electron microscope, there has been used a "wobbler". The wobbler is designed to vary the tilting angle of the irradiation electron beam on the specimen at a proper frequency using a deflection coil so that a magnified image on the imaging plane swings if the electron beam is not focused finely on the specimen. The microscope observer conducts the focus adjustment by operating the wobbler so that the swing of the image vanishes. This focusing method is widely used because of its simplicity. However, it requires of the naked eye to detect a swinging image, which is difficult for a dark image, and depends much on the effort of the observer.
With the intention of overcoming the above-mentioned problems, there has been proposed a focusing system as disclosed in Japanese Patent Examined Publication No. 49-22575, in which several electron beam sensors are arranged on the imaging plane and a swing of image caused by the wobbler operation is detected electrically by summing the absolute values of output deviations produced by the sensors. This electrical focus adjustment system negates the need for the detection of a swinging image by the naked eye to some extent, but has a drawback of inaccurate focusing due to inferior signal S/N property. The system operates to deflect the irradiation electron beam periodically so as to sample the sensor output signals, and in order to obtain noise-free output signals each electron beam sensor is followed by a narrow bandpass filter which admits only a component of frequency equal to the electron beam deflection frequency. The sensor output signal, however, includes not only the fundamental frequency pertaining to the swing of image but also many frequencies caused by the spatial frequencies of the pattern of image. Using only the fundamental frequency makes the net signal amplitude smaller and does not much improve the S/N property, particularly for a specimen image produced by a weak electron beam, and therefore accurate focusing is not achieved. Moreover, the above-mentioned electrical focusing system estimates the fineness of focus in terms of the sum of the absolute values of sensor output deviations, providing little difference of evaluated values between in-focus and out-focus conditions of a low-contrast specimen having smaller variation in concentration, and therefore accurate focusing is not achieved.